Since the early 1950's, when Voorhees observed an essentially thrombus-free silk thread in a prosthesis explanted from a dog, various polymeric materials have been evaluated for use in porous vascular prostheses. Most commercially available synthetic vascular grafts presently in use are made from either expanded polytetrafluoroethylene (e-PTFE), or woven, knitted, or velour design polyethylene terephthalate (PET or Dacron). Dacron grafts are normally used for large vessel replacement (12-22 mm), whereas e-PTFE is generally used for intermediate diameters (6-12 mm). These conventional prosthetic vascular grafts do not permit unrestricted vessel ingrowth from surrounding tissue due mostly to ingrowth spaces that are either too narrow or discontinuous. When used for smaller diameters, these grafts often fail early due to occlusion by thrombosis (fibrous tissue build up) or kinking, or at a later stage because of anastomotic or neointimal hyperplasia (exuberant muscle growth at the interface between artery and graft). Compliance mismatch between the host artery and the synthetic vascular prosthesis, which may result in anastomotic rupture, disturbed flow patterns and increased stresses, is thought to be a causative factor in graft failure. Other causative factors may include the thrombogenicity of the grafts or the hydraulic roughness of the surface, especially in crimped grafts.
In attempts to facilitate ingrowth, the pore size of commercial e-PTFE grafts has been increased. Due to the irregular structure of the pores (between the nodes and internodular fibers), the available ingrowth spaces rapidly narrow down to sub-arteriole dimensions. Various researchers have produced “foam type” grafts, and although compliance matching was achieved to some extent by some, the structures obtained by them have certain disadvantages that prohibit or inhibit the ingrowth of connective tissue. These disadvantages include closed external and/or internal surfaces, closed or semi-closed cell structures with little or no inter-pore communication, and irregularly shaped and sized pores due to irregular filler materials used in the processes.
Because of their unique combination of physical, chemical and biocompatible properties, polyurethanes have been studied and used in medical devices for over thirty years. Enzymatic hydrolysis, auto-oxidation, mineralization, and biologically induced environmental stress cracking of polyester- and polyetherurethanes have led manufacturers of medical polyurethanes to develop more specialized formulations to prevent these occurrences. These new generation polyurethane elastomers are being increasingly accepted as the biomaterials of choice in most applications, especially those requiring compliance. It is not surprising, therefor, that many researchers have used various polyurethane compositions (and other elastomers) to produce vascular grafts. Salt casting, phase inversion, spraying, and replamineform techniques have been used to produce sponge-like structures containing ill-defined pores, while filament winding and electrostatic spinning result in the formation of filamentous or fibrous structures. In the production of many of these devices, researchers have been able to approximate the compliance of natural blood vessels by careful manipulation of the process variables. Nevertheless, the performance of these experimental grafts is generally unsatisfactory. This indicates that compliance matching alone does not result in the desired healing patterns.
There is thus a need or desire in the vascular prosthesis industry for a vascular graft having a well-defined pore structure in its walls to allow uninterrupted ingrowth of connective tissue into the walls of the prosthesis, wherein the problems of compliance mismatch are overcome.